Referring to FIG. 1, which is a block diagram of a conventional wireless communications transceiver system, an incoming input signal having a radio frequency carrier modulated by an incoming baseband signal component, for example an audio signal component, is received by an antenna 10 and is conducted through a diplexer 11 to a low noise amplifier 12 where it is amplified. The frequency of the amplified radio frequency signal is converted to an intermediate frequency signal by a radio frequency mixer 14 and the intermediate frequency signal is amplified by a variable gain amplifier 16. The amplified intermediate frequency signal is converted to a low frequency signal by an intermediate frequency mixer 18 and the low frequency signal is amplified by a variable gain amplifier 20. The amplified low frequency signal is supplied to a demodulator 22 where it is demodulated to detect the incoming audio component and the incoming audio component is amplified by an audio amplifier 24 and supplied to a speaker 26 from which a user hears the incoming audio component carried by the incoming radio frequency signal.
This conventional wireless communications transceiver system also has a microphone 28 that receives an audio input from a user and develops an audio signal that is amplified by an audio amplifier 30 and modulates, in a modulator 32, a low frequency signal. This modulated low frequency signal is amplified by a variable gain amplifier 34 and the amplified low frequency signal is converted to an intermediate frequency signal by an IF mixer 36. The intermediate frequency signal is amplified by a variable gain amplifier 38 and the amplified intermediate frequency signal is converted to a radio frequency signal by an RF mixer 40. The radio frequency signal is amplified by a driver amplifier 42 and the amplified radio frequency signal is conducted through diplexer 11 to antenna 10 for transmission.
Generally, mixers in communications equipment have a fixed gain. Examples of such mixers are Gilbert cell mixers, FET mixers, and diode mixers.
Referring to FIG. 2, which is a circuit diagram of a conventional Gilbert cell mixer included in a conventional wireless communications transceiver system, this mixer circuit includes a mixer core circuit or mixer quad composed of a plurality of transistors 50, 52, 54, and 56. The mixer quad provides a switching action at a rate equal to the frequency of a local oscillator signal applied at a local oscillator port LO P and LO N. This switching action, responsive also to two current signals supplied from the collectors of a pair of transistors 58 and 60 in an input section circuit, converts the frequency of an input signal applied at an input port IN P and IN N to a different frequency output signal that is produced at an output port OUT P and OUT N. The input section circuit also includes a pair current sources 62 and 64 connected in series with transistors 58 and 60, respectively, and a resistor 66 connected between the emitters of transistor 58 and 60.
In a transmitter application, the signal applied at input port IN P and IN N is an intermediate frequency signal and the local oscillator switching action produces a radio frequency signal at output port OUT P and OUT N. In a receiver application, the signal applied at input port IN P and IN N is a radio frequency signal and the local oscillator switching action produces an intermediate frequency signal at output port OUT P and OUT N. The input section circuit serves to translate a voltage input signal into two current signals supplied to the mixer quad from the collectors of transistors 58 and 60.
In the absence of any voltage difference between the base of transistor 58 and the base of transistor 60, the collector currents of these two transistors are essentially equal. Thus, a voltage applied to the local oscillator port LO P and LO N results in no change of output current. Should a small DC offset voltage be present at the input port IN P and IN N (e.g., due to a mismatch in the emitter areas of transistors 58 and 60), this will only result in a small feed through of the local oscillator signal to the output port OUT P and OUT N which will be blocked by a first filter (not shown). Conversely, if an input signal is applied to the input port IN P and IN N, but no voltage difference is applied to the local oscillator port LO P and LO N, the output currents will again be balanced. A small offset voltage (due now to emitter mismatch in transistors 50, 52, 54 and 56) may cause some input signal feed through to the output port OUT P and OUT N.
As before, this will be rejected by the filter. Thus, it is only when a signal supplied to both the local oscillator port LO P and LO N and the input port IN P and IN N that a signal appears at the output port OUT P and OUT N.
If the loads 68, 70, 72, and 74 are purely resistive and equal, the gain is of the circuit is approximately equal to Z/R where Z is the impedance of each of the loads 68, 70, 72, and 74 and R is the resistance of a resistor 66. Transistors 58 and 60 modulate the outputs of current sources 62 and 64 at the frequency of the signal at input port IN P and IN N and provide a high impedance to the emitters of transistors 68, 70, 72, and 74 of the mixer quad. This high impedance improves the common-mode rejection of the circuit.
Resistor 66 also acts as emitter degeneration that provides increased linearity at the input of the mixer. One measure of mixer linearity is the power gain compression point OP1 dB. This parameter can be measured at the input IP1 dB or at the output OP1 dB. OP1 dB is equal to IP1 dB plus the gain of the mixer.
As indicated, variable gain amplifiers are provided in both the receiver chain and the transmitter chain. The gain in each chain is adjusted based on the levels of the received and transmitted signals and what is needed to maintain appropriate signal levels. As arranged in FIG. 1, the wireless communications transceiver system is more complex than desired and requires more space than desired.
It has been suggested in the past to arrange the mixers with variable gains to improve the facility to adjust the power of a wireless communications system. While variable gain mixers add to the facility to adjust the power of a wireless communications system, typically there is a loss in linearity and a loss in the rejection of harmonics.